Byeong-Ui Moon

Water-in-Water Droplets by Passive Microfluidic Flow Focusing.

We present a simple microfluidic system that generates water-in-water, aqueous two phase system (ATPS) droplets, by passive flow focusing. ATPS droplet formation is achieved by applying weak hydrostatic pressures, with liquid-filled pipette tips as fluid columns at the inlets, to introduce low speed flows to the flow focusing junction. To control the size of the droplets, we systematically vary the interfacial tension and viscosity of the ATPS fluids and adjust the fluid column height at the fluid inlets. The size of the droplets scales with a power law of the ratio of viscous stresses in the two ATPS phases. Overall, we find a drop size coefficient of variation (CV; i.e., polydispersity) of about 10%. We also find that when drops form very close to the flow focusing junction, the drops have a CV of less than 1%. Our droplet generation method is easily scalable: we demonstrate a parallel system that generates droplets simultaneously and improves the droplet production rate by up to one order of magnitude. Finally, we show the potential application of our system for encapsulating cells in water-in-water emulsions by encapsulating microparticles and cells. To the best of our knowledge, our microfluidic technique is the first that forms low interfacial tension ATPS droplets without applying external perturbations. We anticipate that this simple approach will find utility in drug and cell delivery applications because of the all-biocompatible nature of the water-in-water ATPS environment.

Microfluidic conformal coating of non-spherical magnetic particles

We present the conformal coating of non-spherical magnetic particles in a co-laminar flow microfluidic system. Whereas in the previous reports spherical particles had been coated with thin films that formed spheres around the particles; in this article, we show the coating of non-spherical particles with coating layers that are approximately uniform in thickness. The novelty of our work is that while liquid-liquid interfacial tension tends to minimize the surface area of interfaces-for example, to form spherical droplets that encapsulate spherical particles-in our experiments, the thin film that coats non-spherical particles has a non-minimal interfacial area. We first make bullet-shaped magnetic microparticles using a stop-flow lithography method that was previously demonstrated. We then suspend the bullet-shaped microparticles in an aqueous solution and flow the particle suspension with a co-flow of a non-aqueous mixture. A magnetic field gradient from a permanent magnet pulls the microparticles in the transverse direction to the fluid flow, until the particles reach the interface between the immiscible fluids. We observe that upon crossing the oil-water interface, the microparticles become coated by a thin film of the aqueous fluid. When we increase the two-fluid interfacial tension by reducing surfactant concentration, we observe that the particles become trapped at the interface, and we use this observation to extract an approximate magnetic susceptibility of the manufactured non-spherical microparticles. Finally, using fluorescence imaging, we confirm the uniformity of the thin film coating along the entire curved surface of the bullet-shaped particles. To the best of our knowledge, this is the first demonstration of conformal coating of non-spherical particles using microfluidics.

Development of a microfluidic device with integrated high frequency ultrasound probe for particle characterization

A microfluidic flow device incorporating a 200 MHz ultrasound probe has been developed to rapidly characterize micron-sized particles. The device hydrodynamically focuses a particle stream under the ultrasound transducer, where pulse-echo ultrasound is used to probe the passing particles one by one. When the ultrasound wavelength is similar to the particle size, the scattered wave depends strongly on the particle size, and the sound speed and density of the particle and surrounding fluid. Each particle type and size has a unique acoustic signature from which it can be identified. To demonstrate this, polystyrene microbeads with two different sizes were used, 6 or 10 μm. Each particle produced an ultrasound signal, and it was identified as either 6 or 10 μm according to unique features in the ultrasound power spectrum. The ultrasound spectral features agreed with those measured from stationary beads using an acoustic microscope, and also to theoretical predictions. These results show for the first time, a new high-speed method of characterizing micron-sized particles using sound waves with applications towards classifying biological cells.